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POPULATION DYNAMICS OF THE WHEAT STEM SAWFLY, CEPHUS CINCTUS (HYMENOPTERA: CEPHIDAE), IN WHEAT

Published online by Cambridge University Press:  31 May 2012

N. D. Holmes
Affiliation:
Research Station, Agriculture Canada, Lethbridge, Alberta T1J 4B1

Abstract

The effects of the major factors on populations of the wheat stem sawfly, Cephus cinctus Norton, were studied in the field at Lethbridge, Alberta between 1970 and 1977. Fecundity of the females, which contain an average of 32.7 eggs, is influenced by the condition and variety of host plants in which they develop as larvae. Populations of the adults that emerge from infested crops cut 7–9% by the larvae in one year may infest 70–80% of the stems of adjacent crops in the following year. Increased adult densities, however, fail to increase markedly the percentage of stems infested because the adults concentrate at field margins when they reinfest the most preferred stems. Because only one larva can mature in each stem, competition within stems that receive more than one egg causes a major drop in the population but not in the percentage of infested stems cut. Another major drop is caused by host plant resistance, which is influenced in both susceptible and resistant wheats by the weather during the growing season. On an average, all of the larvae die in 28% of the infested stems of two susceptible wheats and in 67% of those of a resistant wheat. The mortality in resistant wheats, however, is generally not sufficient to prevent economic infestations in subsequent years. Parasitism, which was a major factor in only 7 of 36 years in southern Alberta, is increased by weather conditions that cause delayed ripening of the wheat crops. The mortality of mature larvae and pupae averaged 21.3%. It is affected by extremes of moisture and temperature and can be increased slightly by shallow tillage of infested stubble.

Résumé

Entre 1970 et 1977, on a étudié sur le terrain, à Lethbridge (Alberta), l'effet des facteurs principaux sur les populations de cèphes du blé (Cephus cinctus Norton). La fécondité des femelles, qui contiennent en moyenne 32.7 oeufs, dépend de l'état et de la variété de la plante hôte dans lesquelles elles se sont développées au stade larvaire. Les adultes qui émergent des cultures infestées, ayant une proportion de 7 à 9% des tiges coupées par les larves durant l'année, peuvent infester 70 à 80% des tiges des cultures adjacentes, l'année suivante. Cependant, une plus grande densité d'adultes n'a pas pour effet d'accroître de façon marquée le pourcentage de tiges infestées, car les insectes se concentrent sur les pourtours des champs où ils réinfestent les tiges préférées. Etant donné qu'une seule larve peut atteindre la maturité dans chaque tige, la concurrence à l'intérieur des tiges qui reçoivent plus d'un oeuf entraîne une chute marquée de la population, mais non du pourcentage de tiges coupées. Un autre facteur responsable de la diminution de la population est la résistance de la plante hôte qui dépend, tant chez les blés sensibles que résistants, des conditions climatiques durant la période de croissance. En moyenne, toutes les larves ont péri dans 28% des tiges infestées de deux blés sensibles et dans 67% de celles d'un blé résistant. Toutefois la mortalité dans les blés résistants n'est généralement pas suffisante pour empêcher des infestations à incidence économique les années suivantes. Ce parasitisme, qui s'est avéré important sept ans, seulement, sur 36, dans le sud de l'Alberta, est favorisé par des conditions climatiques qui retardent le mûrissement du blé. La mortalité des larves mûres et des pupes atteignait en moyenne 21.3% : elle est favorisée par des conditions extrêmes de température et d'humidité et peut être accrue légèrement par un travail superficiel des chaumes infestés.

Type
Articles
Copyright
Copyright © Entomological Society of Canada 1982

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